Many companies have long desired a constantly available time source that is a reliable national standard with which to regulate timing of their own equipment or for viewing. For example, traffic signal manufacturers desire an inexpensive standard means for aligning traffic signals according to time periods of day so that the signals can modify light intervals with the time of day and changing traffic flow. Computer service companies desire a standard basic continuously updated clock in order to coordinate the activities of computers in various locations.
The National Bureau of Standards has been broadcasting time information on standard frequencies for many years from stations in Ft. Collins, Colorado and Kauai, Hawaii. Companies have attempted to market a clock that could receive and decode this signal; however, the signals are relatively weak and therefore are subject to noisy reception. Thus, radio signal receiver clocks that have been known in the prior art may fail to lock on to the signal for long periods of time, or adopt an incorrect timebase. Moreover, many of the applications requiring precise time cannot afford to use an expensive clock.
The primary objective of this invention is to provide an inexpensive, highly accurate clock that is periodically updated by a received, broadcast time reference signal. To understand the invention, it is first necessary to understand some of the details of the clock signal that is broadcast.
The National Bureau of Standards broadcasts continuous signals containing time, date and other information on high frequency radio stations WWV in Ft. Collins, Colorado, and WWVH located in Hawaii. The radio frequencies used are 2.5, 5, 10, 15, and 20 Mhz. All frequencies carry the same program, but because of changes in ionospheric conditions, different frequencies are more easily received at different times of day. The time being broadcast is on the universal time scale also known as Coordinated Universal Time (UTC), formerly Greenwich Mean Time. This time scale is based on atomic clocks with corrections made for the rotational variations of the earth. The specific hour and minute transmitted in the broadcast and mentioned in the audio portion of the broadcast is that corresponding to the time zone centered around Greenwich, England. The UTC time differs from local time only by an integral number of hours in most countries including the United States of America. The UTC time announcements and transmissions are expressed in the 24 hour clock system i.e. the hours are numbered beginning with zero hours at midnight through 12 hours at noon to 23 hours, 59 minutes just before the next midnight.
As noted above, the National Bureau of Standards broadcast uses a carrier at 2.5, 5, 10, 15, and 20 Mhz with a 1000 Hz amplitude modulating tone burst to signal the beginning of each minute on Colorado station WWV, and a corresponding 1200 Hz amplitude modulating tone burst on Hawaii station WWVH. A 100 Hz subcarrier contains binary coded decimal (BCD) signals that supply day of the year, hour and minute information. Complete BCD information in the form of a frame is transmitted each minute. This information is encoded by pulse width modulation of the 100 Hz subcarrier. The data rate is one baud per second, where each symbol is a 0, 1 or position marker. Within a time frame of one minute, enough pulses are transmitted to convey, in BCD, the current minute, hour and day of the year. Two BCD digits are needed to show the hour and the minute (00-23 and 00-59), and three digit groups are needed to show the date (001-366). The BCD time information is updated every minute. The BCD signals also have data providing a correction for periodic variations in the speed of the earth's rotation and information indicating whether daylight savings is in effect.
The most frequently transmitted signals on WWV and WWVH are pulses that mark the seconds of each minute (except the 29th and 59th second pulses of each minute, which are omitted completely), referred to hereafter as ticks. The first pulse of each hour is an 800 ms pulse of 1500 Hz. The first pulse of each minute is an 800 ms pulse of 1000 Hz (WWV) or 1200 Hz (WWVH). The remaining second pulses, or ticks, are brief audio bursts (5 ms pulses of 1000 Hz or 1200 Hz) that resemble the ticking of a clock. All pulses are commenced at the beginning of each second, and are given by means of double side band amplitude modulation. Each seconds pulse (or tick) is preceded by 10 ms of silence and followed by 25 ms of silence to avoid interference.
A more complete description of the signal format may be found in the National Bureau of Standards special publication 432, incorporated herein by reference. In this disclosure, reference is frequently made to Station WWV; however, this clock also receives Station WWVH, automatically selecting the first available signal of acceptable quality. Reference is also made to 1000 Hz, the WWV broadcast frequency; this clock also receives and samples the 1200 Hz signal of WWVH.
Because the earth's speed of rotation may vary, the use of leap seconds is occasionally necessary, perhaps once a year, to keep the broadcast time signals (UTC) within .+-.0.9 seconds of the earth related time scale. The addition or deletion of exactly one second occurs at the end of the month. The system herein has the capability of detecting leap seconds; therefore, a brief summary of the meaning of leap seconds is disclosed herein. When a positive leap second is required, an additional second is inserted beginning at 23 h 59 m 60 s of the last day of the month and ending at 0 h 0 m 0 s of the first day of the following month. In this case, the last minute of the month in which there is a leap second contains 61 seconds. Assuming unexpected large changes do not occur in the earth's rotation rate, it is likely that positive leap seconds will continue to be needed about once a year. If the earth should speed up, a negative leap second is deleted. In this case, the last minute of the month would have 59 seconds.
Prior efforts have been made to provide a clock signal receiver that receives the radio broadcast signals described above, such as the system disclosed in U.S. Pat. No. 4,582,434. However, the system in that patent is subject to locking onto a noise-heavy signal rather than the desired signal; this is a major fault because the NBS signal transmissions are subject to serious noise problems. Further, the '434 patent lacks a sufficiently reliable method of verifying accurate data reception.